This blog post may be a little more vague than my others, as I'm not sure exactly how much detail I want to get into regarding my personal experience of sexuality as I grew up fundie Calvinist. But I do think it's an important subject, and I've seen so many questions on FJ about different fundies and how they understand and experience sex. I can't speak to each family, of course, but I'm reasonably certain that my upbringing was not far from the fundie norm.
So to start: sex education. We didn't have any. Or, more accurately, when I was 11 years old I was given a pamphlet called Almost 12 by Kenneth Taylor, which is still available on Amazon. It had very minimal information, and my mother glued black construction paper over the two-page illustration that, I assume, was of naked people. I had no discussion of the book with my parents: I think my mother asked if I had any questions, and I said no. Of course I said no; I was mortified.
So I had some idea that "sex" involved putting a penis inside a vagina (no idea of any actual mechanics, positions, etc.). Also "petting" and kissing before marriage was bad and wrong, and that courtship was the God-approved way to find a marriage partner. I read so many books on courtship, although I blessedly escaped Josh Harris' semenal work (yes, I made a pun, sue me). Josh Harris was actually a second wave for us; my younger siblings read his book, I think. I remember some of the books I read: Jeff McLean: His Courtship (Castleberry), Dear Princess (Landis), and some crap by Doug Wilson.
The church I grew up in had no position on courtship or dating. Premarital sex was taboo, of course. The pastor and his wife taught abstinence courses in the local schools, so they were pretty adamant about waiting until marriage. It was my parents who pushed the idea that daughters should submit to their fathers in everything, including choice of marriage partner. They had a lot of books on Dominionism and Reconstructionism, which are so anti-women's rights it's not even funny. I devoured those books; I'm not sure what that says about my self-esteem at the time.
When I started getting periods, I told my mother that I was bleeding. I knew what it was, but I was embarrassed to tell her. She said "Oh, you must be getting your..." and trailed off, and my idea that menstruation was embarrassing was reinforced. I knew where the pads were: we didn't use tampons as that might break your hymen, which was your proof of virginity.
Some of my younger sisters ended up coming to me for their periods/problems with their periods, and I helped them as best I could. I studied enough science to have a pretty good idea about what was going on during a woman's cycle during my teen years. I still had no real understanding of sex. My doctor suggested putting me on birth control for my irregular, painful periods, and I wouldn't take birth control because it "could cause abortions." (I knew I couldn't get pregnant without having sex, but I still saw BC as evil).
From a very young age, I had realized that rubbing myself in certain areas felt good, and I did that for years. I had a sense that it was not an approved activity, so I did it privately, with feelings of guilt that I did not understand. I did not know there was a word for it, or that it was a sexual activity, or what an orgasm was when I first had one. But when I was 16 I came across the word "masturbation" and looked it up secretly in the dictionary. I was horrified to find that I had been engaging in a sexual activity outside of marriage. I wasn't sure that I could be considered a virgin anymore, and I was afraid that no good Christian man would want me after what I had done. I went cold turkey off the masturbation for years, riddled with secret shame.
Even after I started becoming less fundie, it took years for me to let go of the sex guilt that I had grown up with. Now I embrace my sexuality. I've gone from thinking "penis" is a bad word to writing erotic stories and selling them online, and I definitely know how the Legos fit together, haha. And I'm happy to say that my siblings, for the most part, also refused to stay repressed.
If there's a moral to this story, it's probably that if you have kids, you need to help them understand sex and sexuality. Make sure they know about consent, protection, STDs, pregnancy, anatomy, periods, masturbation, nocturnal emissions, same-sex attraction, bisexuality, asexuality, porn, and whatever else they might need to know to be safe, sane, consenting, kind, knowledgeable people in their sex lives. I look back at my lack of education and the sexual taboos I grew up with, and wonder how I got out of it without even more significant damage. (I do have some issues that I'm not comfortable sharing on a public forum, but I've read things about other people's difficulties that are much worse).
I know that sexual repression can occur without fundamentalist Christian beliefs. I think my family would have been secretive and embarrassed about these subjects even without the fundamentalism. But the fundamentalism added a huge dose of guilt and shame to everything, along with homophobic religious beliefs and forbidding any sexual outlet outside of marriage. It's more difficult to let go of certain teachings when you're afraid of God and Hell.
I hope this was helpful, or informative, or mildly interesting. Fundamentalist Christianity can go fuck itself.

Some fundies have changed over the years. Some changes are pretty minor while others are rather dramatic. For example, Zsuzsanna Anderson used to crap all over medicated hospital births but opted to go to the hospital when she was in labor with #10 and loved every minute of her epidural. Erika Shupe has sent her children to public school. The Jeubs have as well.
Which fundie change shocked you the most? Which change do you want to know more about? It would be pretty interesting to make a list of the fundie transformations. But kind of hard to do.

I have noticed that fundies speak differently in casual conversations. Instead of talking about going out for a walk, they will say things like "It was such a privilege to spend time outside in the Lord's creation". They also tend to say things like "such a blessing" "joy" "pleasure" a lot. Let's keep a running list going of this "Fundie Language"! What do you usually hear them say?